In a Wavelength Division Multiplexing (WDM) communications network it is essential to monitor the wavelength of signals from different nodes within the network.
In the past optical spectral analyzers and multi-channel optical wavelength meters were used to monitor the wavelengths of various signals at nodes throughout the WDM communications network. However both optical spectral analyzers and multi-channel optical wavelength meters use mechanical scanning mechanisms which respond slowly to wavelength change and are expensive.
Phased Array Waveguide Gratings (PAWGs) which eliminate mechanical scanning mechanisms were then introduced to be used in wavelength monitoring circuits. These circuits utilize the passband cross-over property of the PAWG to monitor the wavelength of various signals at nodes throughout a WDM communications network. However, since there are two PAWG passband peaks inside each channel spacing and the crossover point is very difficult to control, the wavelength range is usually less than 50% of the signal channel spacing resulting in poor system performance.